Upgrade tests to Junit5

See gh-14737

spring-boot-samples/spring-boot-sample-session-webflux/src/test/java/sample/session/SampleSessionWebFluxApplicationTests.java

@@ -18,15 +18,13 @@ package sample.session;
 
 import java.util.Base64;
 
-import org.junit.Test;
-import org.junit.runner.RunWith;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.web.server.LocalServerPort;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseCookie;
-import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.web.reactive.function.client.ClientResponse;
 import org.springframework.web.reactive.function.client.WebClient;
 
@@ -37,7 +35,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Vedran Pavic
  */
-@RunWith(SpringRunner.class)
+
 @SpringBootTest(properties = "server.servlet.session.timeout:2", webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
 public class SampleSessionWebFluxApplicationTests {